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J. C. Wyngaard and O. R. Coté

Abstract

Measurements of the shear production, buoyant production, turbulent transport (flux divergence) and dissipation terms in the budget of turbulent kinetic energy, and production and turbulent transport terms in the temperature variance budget are presented. Direct observations of the surface stress and heat flux over a horizontally uniform site enable presentation of the data in terms of surface layer similarity theory.

The dissipation term, obtained from differentiated hot-wire anemometer signals, agrees with estimates made from the inertial subrange levels of longitudinal velocity spectra with a value of 0.5 for the spectral constant. Under stable conditions dissipation essentially balances shear production, while turbulent transport and buoyant production are of secondary importance. Under unstable conditions, dissipation slightly exceeds the total production, and energy is also lost at a substantial rate due to upward export by the turbulence.

The large imbalance among the measured terms in the energy budget under unstable conditions is discussed. The cause of the imbalance cannot at this point be determined with certainty, but an interesting possibility is that pressure transport is significant under very unstable conditions.

The production rate of temperature variance exceeds its rate of vertical transport by an order of magnitude. Estimates of the universal temperature spectral constant were made with the assumption that temperature variance dissipation and production rates are equal; the average value, 0.8, falls within the range reported by other workers.

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J. C. Wyngaard, O. R. Cote, and Y. Izumi

Abstract

No abstract available.

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J. C. Wyngaard, S. P. S. Arya, and O. R. Coté

Abstract

It is shown that although Coriolis forces cause large production rates of stress in a convective planetary boundary layer, there is a control mechanism, involving mean wind shear which prevents stress levels from becoming large. Higher-order-closure model calculations are presented which show that the stress profiles are essentially linear, regardless of wind direction, providing the geostrophic wind shear vanishes and the wind speed jump across the capping inversion is negligible. It is shown that it will he very difficult to verify these predicted stress profiles experimentally because of averaging time problems. A simple two-layer model is developed which leads to geostrophic drag and heat transfer expressions in fairly good agreement with Wangara data.

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K. S. Rao, J. C. Wyngaard, and O. R. Coté

Abstract

The effects of an abrupt change of surface roughness on the mean flow and turbulence structure in the neutral surface layer are numerically investigated by a higher-order turbulence closure theory, which includes dynamical equations for Reynolds stresses and the viscous dissipation rate. The closed system of governing equations, together with the specified initial and boundary conditions, is solved by an explicit finite-difference method on a digital computer.

The numerical model predicts the distributions of mean wind, shear stress, turbulent energy and other quantities, with no a priori assumptions regarding the distributions of any of these variables in the transition region. The distributions of the nondimensional wind shear, the dissipation and mixing length scales, and the ratio of stress to turbulent kinetic energy are shown to differ significantly from their equilibrium flow variations.

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J. C. Wyngaard, O. R. Coté, and Y. Izumi

Abstract

Equations for the conservation of Reynolds shear stress and the two components of heat flux (velocity-temperature covariance) in the homogeneous atmospheric surface layer are derived. The behavior of the production and turbulent transport (flux divergence) terms in each budget is determined directly from measurements obtained over a wide range of stability conditions during the 1968 Kansas field program of AFCRL.

The data are presented in the dimensionless form suggested by Monin-Obukhoy similarity theory, and follow universal functions quite well. The theory is extended to the “local free convection” regime which exists under very unstable conditions, and specific power law forms are predicted. Several of these are verified and values are given for the proportionality factors in the power laws.

The flux divergence terms are small, implying that in each budget the local production and destruction rates are in balance. The third moments which represent the vertical fluxes of stress and heat flux are small under stable conditions, but are large on the unstable side and indicate that turbulence transfers shear stress and heat flux upward at a velocity u *.

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J. C. Kaimal, J. C. Wyngaard, D. A. Haugen, O. R. Coté, Y. Izumi, S. J. Caughey, and C. J. Readings

Abstract

Results from a boundary layer experiment conducted over a flat site in northwestern Minnesota are discussed. Wind and temperature fluctuations near the ground were measured with AFCRL's fast-response instrumentation on a 32 m tower. Measurements between 32 m and the inversion base zi were made with MRU probes attached at five different heights to the tethering cable of a 1300 m2 kite balloon. The daytime convective boundary layer appears to be well-mixed with evidence of significant heat and momentum entrainment through the capping inversion.

The spectra of velocity components are generalized within the framework of mixed-layer similarity. The characteristic wavelength for w increases linearly with height up to z = 0.l zi following free convection prediction, but approaches a limiting value of 1.5 zi, in the upper half of the boundary layer. The characteristic wavelengths for u and v are maintained at approximately 1.5 zi down to heights very close to the ground. This limiting wavelength corresponds to the length scale of large convective elements which extend to the top of the boundary layer.

The behavior of the temperature specra above 0.l zi cannot be generalized in the same manner. Below that height the θ spectra follow behavior observed in the surface layer; z = 0.1 zi is also the upper limit for the free convection predictions of the w and θ variances.

The high-order moments and the structure parameters reveal the strong influence of entrainment at heights above 0.5 zi.

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